Before I buy a new alternator...

[Typhoon4Fun]

First week with our new to us CD25D! I'll have to follow up with some pictures and an update on projects/improvements so far.

My brother in law was helping me with some docking practice the other night and we ran the motor for about an hour and a half. Yanmar 1GM. I checked the battery voltage after and thought it was funny that it was only reading ~12.3 immediately after shutting off the motor. I went back out with a multimeter and found no change in voltage reading at the battery with the motor at ~2000 rpm. Put the multimeter to the alternator and read ~12.3 (probably just reading the battery voltage?). Seems easy enough to swap out the alternator - I've already had it off to access the cooling system zinc - but before I go and buy a new alternator, is there anything else I should try?

Appreciate any input! We've had a Typhoon and a TySr before this so the diesel is new to me.

[wikakaru]

It sounds like an alternator problem, but before you swap out the alternator you should check that the power from the ignition switch is getting to the terminal on the back of the alternator that provides power to the field wirings.

You may also want to make sure that the alternator is a stock unit with an internal regulator and that it is not the type that uses an external regulator. If that were the case then it could be a problem with the external regulator not the alternator. I'm guessing that on a boat as small as a CD25D that none of the previous owners would have gone to the extra expense and complication of an external regulator, but you never know.

Also, if you have a tachometer and it is reading properly, that output comes from the alternator so it would indicate the alternator may be OK.

Smooth sailing,

Jim

[Typhoon4Fun]

Thank you for the response, Jim! Learning a lot about alternators this AM...

Based on other internet sleuthing, it seems the 1GM may have a sensor that reads a magnet on the flywheel; i.e. tachometer reading does not come from the alternator.

I had no idea about the field wiring. Now to wrap my head around testing for continuity there. I'll report back.

Tom

[Steve Laume]

Any good auto electric shop could check the functioning and rebuild your alternator for you, if necessary.

[Typhoon4Fun]

Pleased to report a happy ending!

Jim, your comment about field wiring made it clear I needed to learn a bit more about alternators. When I was trying to diagnose the alternator, I only noted 2 wires connected to it. Reviewing diagrams and multiple other forum posts made me realize there should have been more, and I remembered that when I moved the alternator out of the way to access the zinc, I was puzzled by a little plastic box with wires leading to it. Did not study it closely as I was preoccupied with the zinc task but assumed it was a junction box or something; then it clicked that it was maybe a plug! Went back to the boat last night and sure enough, somehow the wiring from the ignition key had worked its way loose from the alternator. In any case, I plugged it back in and it now appears to be charging fine - voltage at the battery jumped right up while racing the motor. I'll return once more tonight to run the motor longer and make sure both batteries get all charged up.

Steve, hopefully I am in the clear for now but good to know!

Thanks!!